The development of new methods for the regio- and stereospecific synthesis of highly substituted heterocycles (dihydro- and tetrahydro- pyrans, furans, piperidines and pyrroles) is proposed. Successful achievement of the proposed chemistry should provide a novel strategy for the synthesis of general classes of organic molecules with demonstrated biologically activity (antiviral, antibiotics, etc.). Unsaturated heterocycles (furan, pyranyl, pyrrole, pyridinyl) will be pi- complexed to transition metal moieties to provide cationic eta4-furan, eta5-pyranyl, eta4-pyrrole, and eta5-pyridinyl species that will be reacted sequentially with nucleophiles and electrophiles in order to prepare highly derivatized oxygen and nitrogen heterocycles in a regio- and stereodefined manner. The metal template in the pi-complexes serves a number of useful roles. It will be used to 1) stabilize otherwise unstable species 2) active unsaturated heterocycles for reaction with nucleophiles, and 3) control the stereochemistry and regiochemistry of the introduction of various organic units to the heterocycle core.